Over the years, many schemes have been developed in order to determine the location of a portable or mobile unit within a defined area or to determine the distance of the mobile unit to a predetermined location in the aforementioned area.
An example of one class of such a system is described in U.S. Pat. No. 3,611,368. This patent shows an electronic distance finder ranging system that uses resonance between two transmitting stations as a distance sensitive factor. Also in this class is U.S. Pat. No. 4,113,382, which concerns a system for measuring the distance between a master transmitting station and a slave station by determining the transmit time of a pulse of optical or electromagnetic radiation between two locations.
The prior art also includes U.S. Pat. No. 4,297,700 which describes a system for measuring distance by transmitting a VHF wave train incorporating a complicated digital ranging code, and requires a very wide bandwidth for better resolution. Further, U.S. Pat. No. 4,698,781 concerns an electronic system wherein signal arrival times or bearings are used to provide distances from a portable interrogation unit to a specified location on a field encompassed by triangularly-disposed slave and master transmitting positions.
Another one-dimensional (distance) measuring system is described in U.S. Pat. No. 4,757,315, wherein sub-radio frequency comparisons of phase differences are made, and requires a transmitter/receiver at the end of each location to which a range measurement is desired.
In all of the above-described prior art schemes, it is required that the mobile or portable unit both receive signals from and transmit signals to fixed stations. Thus, all the mobile or portable units must carry relatively large power supplies to power their mobile transmitters, and additional transmitter frequency allocations are required for additional mobile units.
In another class of prior art ranging or location-determining systems, where the mobile station has no transmitter, U.S. Pat. No. 3,868,692 describes a golf yardage finder which allows golfers to determine the distance of their next shot while approaching a green. This system uses signal strength measurements to make this determination, and is one-dimensional. It also is rather complicated in that it must deal with 18 different frequencies, one for each hole in a standard golf course. Also, in this latter class, U.S. Pat. No. 4,703,444 concerns a digital system to produce digital ranging signals. Each of a plurality of fixed transmitters transmits a repetitive ranging signal. The ranging signal pulses from the respective fixed transmitters are sequentially staggered in a predetermined order to define repetitive sets of sequentially spaced ranging signal pulses which may be processed by a portable receiving unit to determine the ranging information desired.
Unlike the first described class of prior art ranging systems that require a portable unit to both transmit and receive rf or light energy signals, the present invention only utilizes a receiver carried by the portable or mobile unit. This lessens the weight, complexity, and expense of the system.
Also, in contrast to the last described class of prior art ranging systems, the present invention does not rely on inherently inaccurate signal strength comparisons or complicated ranging coded signals to produce the desired ranging information. In fact, no prior art system known to the inventor describe a system wherein only uninterrupted continuous wave (cw) signals are transmitted by fixedly located rf transmitting stations to a mobile or portable receiving-only unit which compares the phase differences of the cw signals to provide the desired location and ranging information.
The important phase comparison feature of the present invention is, however, used in what is well known as the OMEGA Navigation system. In the OMEGA system, the various fixed location transmitters do not transmit continuously, but rather transmit alternately so that the receiving unit must remember the phase of each of the different received signals to allow a comparison to be made.
In contradistinction to the prior art, the present invention is a dependable, accurate and ultra-functional solution to the problems raised in the prior art structures.